Some Parasitic Diseases of Blue Crab

This is in need of some updating and will soon be supplanted with a more
rigorous and thorough review. (Shields, J.D. 1997. The potential effects of
outbreaks of parasitic diseases on the blue crab fishery of Virginia. 2ndVirginia Eastern Shore Natural Resources Symposium,
The Eastern Shore Institute, Exmore, VA. TESI Publication #4: 23-29.)

Hematodinium in Hemolymph

Vermiform plasmodium of Hematodinium perezi. This stage is diagnostic. The plasmodia are also motile. The cells in the background are hemocytes.

Schizogony of plasmodium. The plasmodia divide via budding to produce more plasmodia, and via schizogony to produce trophonts. The mass of cells in the center of the field represents the schizont.

Introduction and Summary of Disease Agents

Some Parasitic Diseases Of the Blue Crab

Several disease agents have occurred in outbreaks or epizootics in blue crabs
from Maryland and Virginia (Table 1). In general, the
outbreaks are localized to specific bays or regions, but widespread epizootics
are known to occur, especially along the seaside locations of the Eastern Shore.
In most cases the causative agents were virtually unknown until the occurrence
of the epizootics (e.g., Sprague & Beckett, 1966, Johnson, 1976; Couch,
1983). In fact, we know little about the conditions that lead to the
epizootics.

While eight viruses have been reported from blue crabs (Johnson, 1983), four
are known pathogens documented in epizootics from Chesapeake Bay or Chincoteague
Bay. The pathogenic viruses typically live in hemocytes, or epithelial cells,
and are associated with significant mortalities during outbreaks. Infected crabs
are lethargic, susceptible to stress-induced mortality (e.g., capture, and
handling), and often show signs of tremors or paralysis, or even blindness (CBV)
(Table 1). Transmission experiments with the viral agents were undertaken via
injection of virus-laden tissues. The reolike virus (RLV) may have an extremely
short, and acute patent period; infection experiments resulted in mortalities in
just 3 days (Johnson & Bodammer, 1975)! The other pathogenic viruses had
patent periods ranging from 2 weeks to 2 months. Transmission in nature has not
been examined, but the short duration of infection for RLV indicates that it
could represent a significant problem to the soft-shell crab industry during
epizootics.

Vibrio bacteria are ubiquitous in the marine environment, and several
invertebrates are infected by or passively transport the disease agents.
Vibrio parahemolyticus has been documented in outbreaks in shedding
facilities for soft-shell crab where it can be a significant source of
mortality. The bacterial pathogen invades the hemolymph through abrasions in the
cuticle of the crab, and multiplies in the nutrient-rich environment of the
crab’s blood. Few studies have addressed transmission, or prevalence of the
Vibrio bacteria in blue crabs, yet the disease may have an effect on the
lucrative soft-shell industry.

Two egg parasites can be found in the high salinity waters of the region. The
fungus Lagenidiumcallinectes can assimilate large numbers of crab
eggs in infected clutches. In some cases, most of the egg clutch can be
destroyed by the fungus. It is widespread and can tolerate moderately low
salinities (Rogers-Talbert, 1948). The nemertean worm Carcinonemertes
carcinophila can ingest relatively large numbers of eggs during development.
On other crab species, epizootics of nemertean worms have virtually wiped out an
entire year’s broodstock (Shields et al., 1991; Kuris et al., 1991), and can
impact significantly on a fishery. For the blue crab, the freshwater influence
of the estuarine environment may limit the spread of the worm and curtail
potential outbreaks in the region (as for C. mitsukurii, Shields &
Wood, 1993). In addition, while both egg predators can occur in relatively high
prevalences in the region, their overall effect may be limited by the high
fecundity of their host.

Two parasites can affect the general quality of the meat of the blue crab.
The microsporan Amesonmichaelis causes severe muscle lysis that
results in a condition known as "cotton crab." Crab meat infected with the
microsporan is cottony in texture, and poorly flavored. The parasite can be
transmitted via cannibalism; and since as much as 25% of a blue crab’s diet is
other blue crabs, it is surprising that the parasite is only found at low
prevalences (<1%, Shields, pers. obs.). Pepper spot disease is the other
condition that affects the quality of crab meat. It is an unusual condition
brought about by a hyperparasitic protozoan, Urosporidium crescens, that
infects the trematode Microphallus bassodactylus. Pepper spot occurs when
the trematode cysts are attacked by the protozoan; the cysts become blackened or
melanized. Crabs are not affected by the disease but it can affect the
aesthetics of the meat. Pepper spot is common on the Eastern Shore (>30% of
crabs from some locations). It appears be related to high salinities.

Two highly pathogenic protozoans are known to cause signficiant mortalities
to blue crabs. Gray crab disease is caused by an amoeba, Paramoeba
perniciosa, that invades the connective tissues, and hemolymph of crabs. As
the common name implies, the ventral surfaces of infected crabs turn gray in
color. As in other systemic infections, the hemocytes of heavily infected crabs
are virtually replaced by the trophic stage of the parasite. Crabs become
lethargic and eventually die, or die from stress-related handling. The disease
and related crab mortalities appear centered around the small coastal bays of
the mid Atlantic states. In spring, mortalities in shedding facilities have been
related to this disease (Newman & Ward, 1973; Couch, 1983). Winter
mortalities of crabs have also been associated with high prevalences of the
amoeba. Transmission of the disease remains unknown but we can speculate that
lethargic crabs would fall easy prey to their voracious brethren, and thus,
effect transmission. Alternatively, the amoeba could invade crabs in the winter
while their hosts are buried in the bottom, especially if infected hosts are
dying at this time.

Hematodiniumperezi is an unusual parasitic dinoflagellate that
also occurs in outbreaks. It lives in the hemolymph of crabs and rapidly
proliferates (Shields & Squyars, 2000). Outbreaks of H. perezi
have been reported from the high salinity waters of the lower Chesapeake Bay,
coastal bays in Maryland and Virginia, Georgia, and Florida (Newman and Johnson,
1975; Couch, 1983; Messick, 1994). During epizootics, crab mortality can reach
50% in set pots, and 75% in shedding facilities on the Eastern Shore. One
outbreak showed a prevalence of 100% in juvenile crabs and up to 70% in mature
crabs (Messick, 1994). Infections are generally terminal with crabs dying
metabolic exhaustion and stress-related handling (Shields et al., in review).
Evidence indicates that temperature and salinity play a key role in the
epizootics of the dinoflagellate (Messick & Shields, in prep.). The disease
is most prevalent in the fall (Messick, 1994; Messick & Shields, in
prep.).

Hematodinium perezi or related species has been identified from a wide
range of host species from many geographic regions. On the eastern seaboard of
the USA, the parasite infects the American blue crab, the rock crabs,
Cancerirroratus, C. borealis, and the lady crab,
Ovalipesocellatus (Maclean & Ruddell, 1978). Related
parasites have also been reported from amphipods (Johnson, 1986). Other species
of Hematodinium have caused considerable losses to several other
important crab fisheries, i.e, Tanner crab, the Norway lobster, and the European
rock crab (Meyers et al., 1987; Latrouite et al., 1988; Field et al., 1992; and
others).

Epizootiology

Recent outbreaks of egg predatory nemerteans, rhizocephalan castrators, and
dinoflagellates on crabs and lobsters have seriously affected or even devastated
host populations in California, British Columbia, Alaska and Scotland. The
causative agents were virtually unknown until the occurrence of the epizootics
(e.g., Shields et al., 1989; Meyers et al. 1987). In Alaskan waters,
environmental factors such as hydrographic conditions and seasonal increases in
water temperature (i.e., fjords with shallow sills) appear to have contributed
to the epizootics (Sloan, 1984; Sloan, 1985; Kuris et al., 1991), and models
suggest that closed populations of hosts may be more seriously affected by
parasites than open populations (Kuris & Lafferty, 1992). Circumstantial
evidence points to the apparent reduction in water mixing/flushing in the
isolated fjords as the primary factor affecting host recruitment and prevalence
of diseases (Sloan, 1984; 1985; Kuris et al., 1991).

The Delmarva Peninsula with its shallow lagoons and backwaters may be an
ideal region for the growth and spread of parasitic diseases in the blue crab. I
have speculated that the region possesses a triad of conditions that are
required for an epizootics of pathogenic diseases (Shields, 1994). The
conditions are (1) relatively closed crab populations (i.e., those with little
immigration and emigration of juveniles and adults), (2) relatively high
salinities with, in some cases, little water exchange between the open ocean and
backwaters (i.e., narrow channels with shallow sills and barrier islands/bars),
and (3) stressful conditions for the crab populations (heat and salinity stress,
seasonal hypoxia, seasonal fishing and predation pressure). Many of the
pathogenic agents appear limited to salinities above 15 o/oo and thus may only
be of significance to the crab fisheries located in the small coastal bays of
the mid Atlantic.

Transmission experiments with many of the disease agents have been partially
successful. Most studies rely on artificial methods of infection like injection,
or cannibalism, but water-borne transmission via contact and direct invasion of
the host are probably the major avenues for many of the disease agents.
Cannibalism may be a major route of transmission since crabs are known
cannibals; the vegetative stages of the diseases can frequently survive in
seawater, or dead flesh for several days; and infected crabs often suffer
lethargy and are therefore more susceptible to predation.

In conclusion, epizootics of various pathogens can occur in blue crabs with
some regularity. They are frequently associated with high salinity waters, and,
in some cases, with water temperature. The hydrography of the Delmarva
Peninsula, and other coastal regions of the mid Atlantic may facilitate the
spread of the epizootics by retaining infected crabs, and by focusing the
infectious agents in the dense populations of hosts.

Table 1. Selected pathogenic agents of the blue
crab, Callinectes sapidus, primary source, tissues in which the diseases
occur, and the status of the agent in causing epizootics in crabs.